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  • 1
    ISSN: 0044-2313
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Single Crystal Structure Determinations of the Cubic High Pressure Elpasolites Rb2LiFeF6 and Cs2NaFeF6: Pressure-Distance Paradox without Change of Coordination NumberAt single crystals of metastable high pressure phases of Rb2LiFeF6 (a = 824.4 pm) and Cs2NaFeF6 (a = 873,9 pm) the parameters of the cubic elpasolite structure (Fm3m, Z = 4) were determined by X-ray methods. Compared to the 12L-structures of the normal pressure phases (R3m, hex. Z = 6) only the distances within the 12-coordination, Rb—F = 291.7 resp. Cs—F = 309.9 pm, are compressed by 2-3%. However, the octahedral distances Fe—F = 194.6 pm and Li—F = 217.6 pm resp. Fe—F = 194.9 pm and Na—F = 242.0 pm, are enlarged by 1-4%, though there was no increase in coordination number. This paradoxical behaviour is discussed. Difference Fourier syntheses reveal disorder only for the lithium positions in Rb2LiFeF6, which are 30 pm off-center, corresponding to a splitting of distances Li—F into 188, 247 and 4 × 220 pm.
    Notes: An Einkristallen der metastabilen Hochdruckphasen von Rb2LiFeF6 (a = 824,4 pm) und Cs2NaFeF6 (a = 873,9 pm) wurden röntgenographisch die Parameter der kubischen Elpasolithstruktur (Fm3m, Z = 4) bestimmt. Gegenüber den 12 L-Strukturen der Normaldruckphasen (R3m, hex. Z = 6) sind nur die Abstandswerte in der Zwölferkoordination, Rb—F = 291,7 bzw. Cs—F = 309,9 pm, um 2-3% verkleinert. Aber für die oktaedrischen Abstände Fe—F = 194,6 pm und Li—F = 217,6 pm bzw. Fe—F = 194,9 pm und Na—F = 242,0 pm ergeben sich Aufweitungen von 1-4%, obwohl keine Erhöhung der Koordinationszahl stattgefunden hat. Dieses paradoxe Verhalten wird diskutiert. Nur für die Lithiumpositionen in Rb2LiFeF6 zeigen Differenzfouriersynthesen eine Fehlordnung, die einer Li-Auslenkung von 30 pm aus dem Oktaederzentrum und einer Li—F-Abstandsaufspaltung in 188, 247 und 4 × 220 pm entspricht.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
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